Performance of bipolar diffusion chargers: Experiments with particles in the size range of 100 to 900 nm

Accurate measurement of particle size distribution using electrical-mobility techniques requires knowledge of the charging state of the sampled particles. A consistent particle charge distribution is possible with bipolar diffusion chargers operated under steady-state condition. Theoretical steady-state charge distributions for bipolar charging are well established but recent studies have shown that the performance of particle chargers is a strong function of particle size, particle concentration, ion source, and charger operating conditions. Most of these studies have focused on particles smaller than 100 nm and the applicability of these results for particles larger than 100 nm must be investigated. In this study, experimentally obtained singly-charged and doubly-charged fractions are compared against theoretical predictions for particles in the size range of 100 to 900 nm. The experimental results show that the commercial soft X-ray charger performs as theoretically-predicted over the range of conditions studied while the performance of other commonly used radioactive chargers (⁸⁵Kr and ²¹⁰Po) are dependent on source strengths, flowrates, particle charge polarities, and particle sizes. From measurements of particle residence times and ion concentrations in different test bipolar chargers, prior observations of flowrate-dependent charging fractions can be explained. Additionally, the results from this study are used to determine an acceptable time period for usage of the commercial TSI 3077A ⁸⁵Kr chargers for steady-state charging as a function of flowrate.

Copyright © 2018 American Association for Aerosol Research     

Determination of the Mean Mobility of Aerosol Nanoparticles Classified by Differential Mobility Analyzers

MonteCarlo simulations of diffusive particle trajectories, as well as Stolzenburg's model calculations, have shown that the mean mobility of the particles classified by a differential mobility analyzer (DMA) at a given applied voltage may differ from the theoretical one inferred from the Knutson–Whitby equation if the particles are withdrawn from the tails of the particle mobility distribution. In this case, the true mean mobility, defined as the mean mobility of the particles classified at the specified voltage, can be precisely measured by a second DMA operating in series with the first one (tandem DMA). However, if particles are extracted from the central part of the distribution, their mobility can be correctly measured with a single DMA. Besides showing the importance of the usage of the tandem DMA technique for accurate measurements of mobility, this article provides an analytical expression which, if the mobility distribution of the polydisperse aerosol fed to the DMA is known, allows an accurate estimation of the true (mean) mobility of the classified particles.

Copyright 2014 American Association for Aerosol Research     

An integrative model for the filtration efficiencies in realistic tests with consideration of the filtration velocity profile and challenging particle size distribution

Many well-established models can be applied to calculate the filtration efficiencies. In these models the filtration velocity and challenging particle size are assumed to be known accurately. However, in realistic filtration tests, the filtration velocity has profiles dependent on the filter holder geometry and experimental conditions; the challenging particles have size distributions dependent on the instruments and operation conditions. These factors can potentially affect the measured filtration efficiency and lead to discrepancies with the models.

This study aims to develop an integrative model to predict the filtration efficiencies in realistic tests by incorporating the effects of the filtration velocity profile and challenging particle size distribution classified by a differential mobility analyzer (DMA) into the existing filtration models. Face velocity profile is modeled with fluid mechanics simulations; the initial generated particle size distribution, the particle charging status and the DMA transfer function are modeled to obtain the challenging particle size distribution. These results are then fed into the filtration models. Simulated results are compared with experimental ones to verify the model accuracy. This model can be used to reduce filtration test artifacts and to improve the experimental procedure.

The results reveal that the face velocity upstream the filter exhibits high degree of homogeneity not affecting the filtration efficiency if the filter pressure drop is not very low. The generated particle size distribution and the DMA selection size window could influence the challenging particle size distribution and therefore the measured filtration efficiency.

Copyright © 2017 American Association for Aerosol Research     

Challenges of measuring nascent soot in flames as evidenced by high-resolution differential mobility analysis

Nascent soot particles with mobility diameters ≤10 nm were measured in an ethylene/air premixed flame to shed light on the challenges and potential artifacts affecting studies on soot inception by differential mobility analysis (DMA) techniques. The size distribution functions (SDFs) of particles with charge acquired either naturally or diffusively upon ion seeding were measured at several positions in the flame using rapid-dilution probing and a high-resolution DMA for different values of the ratio of dilution ratio to residence time (DR/Δt). The SDFs are roughly bimodal with a sub-3 nm mode and a larger one that appears either downstream in the flame or for low DR/Δts. Soot nuclei smaller than 3 nm preferentially acquire positive charge, which brings into question the assumption of steady-state charging probability of flame sampled soot nuclei in the bipolar diffusion neutralizer. The approximately polarity-symmetric lognormal SDF of larger particles is attributed to nuclei coagulation. Naturally charged particles increase in number when lowering DR/Δt, suggesting either their collisional charging by flame chemi-ions or particle nucleation by condensation of neutral molecules on ions or both. The critical conditions for suppressing particle coagulation and charge redistribution in the sampling system were not achieved under most conditions, despite the fact that values of DR/Δts were more favorable to such a suppression in the present experiment as compared to other studies in the literature. As a result, the identification of this “asymptotic” regime, which is critical to determine the parent SDFs and the charge state of nascent soot in the flame, is still elusive.

© 2016 American Association for Aerosol Research     

Influence of flame-generated ions on the simultaneous charging and coagulation of nanoparticles during combustion

Flames generate a large amount of chemically and thermally ionized species, which are involved in the growth dynamics of particles formed in flames. However, existing models predicting particle formation and growth do not consider particle charging, which may lead to bias in the calculated size distribution of particles. In this study, Fuchs' charging theory was coupled with a monodisperse particle growth model to study the simultaneous charging and coagulation of nanoparticles during combustion. In order to quantify the charging characteristics of nanoparticles, a high-resolution DMA was used to measure the mobilities of ions generated from a premixed flat flame operated at various conditions. The effect of temperature on ion–particle and particle–particle combination coefficients was further examined. The proposed model showed that the influence of charging on particle growth dynamics was more prominent when the ion concentration was comparable to or higher than the particle concentrations, a condition that may be encountered in flame synthesis and solid fuel-burning. Simulated results also showed that unipolar ion environments strongly suppressed the coagulation of particles. In the end, a simplified analysis of the relative importance of particle charging and coagulation was proposed by comparing the characteristic time scales of these two mechanisms.

© 2017 American Association for Aerosol Research     

Methodology to quantify the ratio of multiple-to single-charged fractions acquired in aerosol neutralizers

A methodology for the quantification of the ratio of multiple- to single-charged fractions acquired in aerosol neutralizers is presented. These quantities are necessary for an accurate monodisperse calibration of aerosol instrumentation. A tandem Differential Mobility Analyzer (DMA) setup is required, with the second DMA scanning the electrical mobility spectra classified in the first DMA. In contrast to previous studies on the quantification of bipolar charge distribution utilizing tandem DMA schemes, the methodology targets at the direct determination of the multiple- to single-charge fractions and does so through the analysis of the raw signal instead of the inverted size distributions, thus circumventing errors associated with the assumptions in the DMA data inversion. The proposed methodology is employed for the characterization of different types of aerosols commonly employed for instrument calibration. Spherical liquid particles (emery oil and dioctyl sebacate) were found to acquire lower multiple charge fractions than those suggested by the commonly employed regression fits of Wiedensohler, which was published in the year 1988 in the Journal of Aerosol Science (vol. 19, pp. 387–389), but still within the range of values reported in the literature. Diffusion flame soot and spark generated graphite particles, produced by a miniCAST 6203C burner and a PALAS DNP 3000, respectively, exhibited higher fraction of multiple charges, in good agreement with previous work on agglomerates. The use of a soft X-ray bipolar charger (TSI 3088) yielded systematically higher multiple fractions of positive charges compared to a ⁸⁵Kr neutralizer (TSI 3077A), confirming the importance of direct photoionization charging on the former.

Copyright © 2016 American Association for Aerosol Research     

Role of oleic acid coating in the heterogeneous uptake of dimethylamine by ammonium sulfate particles

Reactive uptake by ammonium (NH₄⁺) salts is one of the major pathways for the gas-to-particle partitioning of alkyl amines. Recent studies using particles of individual ammonium salts and mixtures with hydrophilic organics have revealed that the degree of amine uptake depends on the phase state of ammonium salts, the particulate water contents and particle viscosity. The role of hydrophobic organic compounds, another important category of particulate organics commonly detected in the ambient atmosphere, in amine uptake remains unknown. Here we report the uptake of dimethylamine (DMA) by ammonium sulfate (AS) particles coated with fresh or ozone-aged bulk oleic acid (OA) at 60%, 30%, and <5% relative humidities (RHs) using an electrodynamic balance coupled with Raman spectroscopy. OA and DMA were selected to represent hydrophobic organics and alkyl amines, respectively. Over 74% of the original NH₄⁺ ions were displaced due to DMA uptake, except those conditioned at <5% RH. On the other hand, the fresh or aged bulk OA coating retarded DMA uptake by preventing the particle surface from effectively accommodating gaseous DMA molecules. Judging from the estimated DMA uptake coefficients, the retardation gradually intensified as the weight percentage of coating increased before leveling off, likely when the particle surface was completely covered by fresh or aged bulk OA. We propose that the accommodation of DMA on the particle coating is the rate-limiting step of DMA uptake. Intensive aging of the OA coating had little effect on the equilibrium particle-phase compositions but retarded DMA uptake.

© 2017 American Association for Aerosol Research     

Accuracy of recovered moments for narrow mobility distributions obtained with commonly used inversion algorithms for mobility size spectrometers

Aerosol mobility size spectrometers are commonly used to measure size distributions of submicrometer aerosol particles. Commonly used data inversion algorithms for these instruments assume that the measured mobility distribution is broad relative to the DMA transfer function. This article theoretically examines errors that are incurred for input distributions of any width with an emphasis on those with mobility widths comparable to that of the DMA's transfer function. Our analysis is valid in the limit of slow scan rates, and is applicable to the interpretation of measurements such as those obtained with tandem differential mobility analyzers as well as broader distributions. The analysis leads to expressions that show the relationship between the inverted number concentration, mean size, and standard deviation and true values of those parameters. For narrow distributions (e.g., for a mobility distribution produced by a DMA with a 1:10 aerosol:sheath air flow ratio) under typical operating conditions, number concentrations and mean mobility obtained with inversion algorithms are accurate to within 0.5% and 1.0%, respectively. This corresponds to mean diameter retrieval errors of 1.0% for large particles and 0.5% for small (kinetic regime) particles. The widths (i.e., relative mobility variance) of the inverted distributions, however, significantly exceed the true values.

Copyright © 2018 American Association for Aerosol Research     

Mass,charge, and radius of droplets in a linear quadrupole electrodynamic balance

Single particle levitation is a key tool in the analysis of the physicochemical properties of aerosol particles. Central to these techniques is the ability to determine the size of the confined particle or droplet, usually achieved via optical methods. While some of these methods are extremely accurate, they are not suitable for all applications and sample types, such as solid or optically absorbing particles. In this work, measurements of the radius, mass, and charge of droplets in a linear quadrupole electrodynamic balance (LQ-EDB) are reported. Using the elastic light scattering pattern produced by laser illumination, a method to determine the radius is described, with an accuracy of as good as ±60?nm and a sensitivity to changes on the order of 10?nm. The effect of refractive index on these measurements is explored by application of the technique to simulated data using Mie theory. In addition to radius, the relative and absolute mass and charge of droplets in the trap is measured from the voltage required to stabilize their vertical position. These measurements are facilitated by stacking multiple droplets in the LQ-EDB and solving the force balance equations to yield both parameters. These approaches are demonstrated through measurements of the evaporation of pure ethylene glycol and pure water droplets, the change in density of an aqueous glycerol solution as water evaporates, and the mass and charge of pure glycerol droplets.

Copyright © 2019 American Association for Aerosol Research     

Characterization of the TSI model 3086 differential mobility analyzer for classifying aerosols down to 1 nm

Measurement systems for particle sizing starting at 1 nm are used to bridge the gap between mass spectrometer measurements and traditional aerosol sizing methods, and thus to enable measurement of the complete size distribution from molecules and clusters to large particles. Such a measurement can be made using a scanning mobility particle sizer equipped with a diethylene glycol growth engine (e.g., TSI Model 3777 Nano Enhancer) along with a condensation particle counter, and a differential mobility analyzer (DMA) appropriate for such small sizes. Previous researchers have used high-resolution DMA (HRDMA) and also the TSI Nano-DMA (Model 3085) in such a scanning mobility particle sizer system. In this study, we evaluate the performance of the recently introduced TSI 1 nm-DMA (Model 3086). The transfer function was characterized using 1–2 nm monomobile molecular ion standards. The same measurements were repeated on a TSI Nano-DMA, with good agreement to previously published values. From the measured transfer function, the resolution of each DMA model was determined as a function of particle size and sheath flow rate. Resolution of the TSI 3086 in the 1–2 nm range was 10–25% higher than the TSI 3085. Measured resolutions of the TSI 3086 were 10–20% lower than theoretically predicted values, whereas those of the Model 3085 were 0–10% lower.

Copyright © 2018 TSI Inc.     

Measuring aerosol size distributions with the aerodynamic aerosol classifier

The Aerodynamic Aerosol Classifier (AAC) is a novel instrument that selects aerosol particles based on their relaxation time or aerodynamic diameter. Additional theory and characterization is required to allow the AAC to accurately measure an aerosol’s aerodynamic size distribution by stepping while connected to a particle counter (such as a Condensation Particle Counter, CPC). To achieve this goal, this study characterized the AAC transfer function (from 32 nm to 3 μm) using tandem AACs and comparing the experimental results to the theoretical tandem deconvolution. These results show that the AAC transmission efficiency is 2.6–5.1 times higher than a combined Krypton-85 radioactive neutralizer and Differential Mobility Analyzer (DMA), as the AAC classifies particles independent of their charge state. However, the AAC transfer function is 1.3–1.9 times broader than predicted by theory. Using this characterized transfer function, the theory to measure an aerosol’s aerodynamic size distribution using an AAC and particle counter was developed. The transfer function characterization and stepping deconvolution were validated by comparing the size distribution measured with an AAC-CPC system against parallel measurements taken with a Scanning Mobility Particle Sizer (SMPS), CPC, and Electrical Low Pressure Impactor (ELPI). The effects of changing AAC classifier conditions on the particle selected were also investigated and found to be small (<1.5%) within its operating range.

Copyright © 2018 American Association for Aerosol Research     

Atmospheric particle composition-hygroscopic growth measurements using an in-series hybrid tandem differential mobility analyzer and aerosol mass spectrometer

The ability of atmospheric particles to absorb water has extensive climate, atmospheric chemistry, and health implications, and considerable effort has gone into determining relationships between particle composition and hygroscopicity. Parallel techniques, in which co-located composition and hygroscopicity measurements are combined to infer composition-hygroscopicity relationships, may not detect the influence of external mixtures. Previous in-line measurements have been limited to single-particle composition or a limited analyte range, and are often non-quantitative and/or offline. Here, we present for the first time in-series, online, quantitative hygroscopicity-composition measurements using a Brechtel Manufacturing, Inc. Hybrid Tandem Differential Mobility Analyzer and an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer. This technique is first verified using laboratory-generated external particle mixtures, then extended to ambient measurements at a seaside sampling side at the Hong Kong University of Science and Technology. The technique successfully separated laboratory-generated particles of differing hygroscopicities and showed promise for atmospheric particles, though high mass attenuation endemic to the HTDMA dual size selection limits application to environments with at least ～14–41 μg/m³ of particulate mass, depending on composition.

Copyright © 2017 American Association for Aerosol Research     

Measuring ultrafine aerosols by direct photoionization and charge capture in continuous flow

Direct ultraviolet (UV) photoionization enables electrical charging of aerosol nanoparticles without relying on the collision of particles and ions. In this work, a low-strength electric field is applied during particle photoionization to capture charge as it is photoemitted from the particles in continuous flow, yielding a novel electrical current measurement. As in conventional photocharging-based measurement devices, a distinct electrical current from the remaining photocharged particles is also measured downstream. The two distinct measured currents are proportional to the total photoelectrically active area of the particles. A three-dimensional numerical model for particle and ion (dis)charging and transport is evaluated by comparing simulations of integrated electric currents with those from charged soot particles and ions in an experimental photoionization chamber. The model and experiment show good quantitative agreement for a single empirical constant, K_cI, over a range of particle sizes and concentrations providing confidence in the theoretical equations and numerical method used.

Copyright © 2018 American Association for Aerosol Research     

Modeling the thermodynamics and kinetics of sulfuric acid-dimethylamine-water nanoparticle growth in the CLOUD chamber

Dimethylamine (DMA) has a stabilizing effect on sulfuric acid (SA) clusters, and the SA and DMA molecules and clusters likely play important roles in both aerosol particle formation and growth in the atmosphere. We use the monodisperse particle growth model for acid-base chemistry in nanoparticle growth (MABNAG) together with direct and indirect observations from the CLOUD4 and CLOUD7 experiments in the cosmics leaving outdoor droplets (CLOUD) chamber at CERN to investigate the size and composition evolution of freshly formed particles consisting of SA, DMA, and water as they grow to 20 nm in dry diameter. Hygroscopic growth factors are measured using a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA), which combined with simulations of particle water uptake using the thermodynamic extended-aerosol inorganics model (E-AIM) constrain the chemical composition. MABNAG predicts a particle-phase ratio between DMA and SA molecules of 1.1–1.3 for a 2 nm particle and DMA gas-phase mixing ratios between 3.5 and 80 pptv. These ratios agree well with observations by an atmospheric-pressure interface time-of-flight (APi-TOF) mass spectrometer. Simulations with MABNAG, direct observations of the composition of clusters <2 nm, and indirect observations of the particle composition indicate that the acidity of the nucleated particles decreases as they grow from ～1 to 20 nm. However, MABNAG predicts less acidic particles than suggested by the indirect estimates at 10 nm diameter using the nano-HTDMA measurements, and less acidic particles than observed by a thermal desorption chemical ionization mass spectrometer (TDCIMS) at 10–30 nm. Possible explanations for these discrepancies are discussed.

Copyright © 2016 American Association for Aerosol Research     

Characterization and Response Model of the PPS-M Aerosol Sensor

The Pegasor PPS-M sensor is an electrical aerosol sensor based on diffusion charging and current measurement without particle collection. In this study, the role and effect of each component in the instrument is discussed shortly and the results from a thorough calibration measurements are presented. A comprehensive response model for the operation of the PPS-M sensor was developed based on the calibration results and computational fluid dynamics (CFD) modeling results. The obtained response model, covering the effects of the particle charger, the mobility analyzer, and both diffusion and inertial losses, was tested in the laboratory measurements with polydisperse test aerosols, where a good correlation between the model and the measured results was found.

Copyright 2014 American Association for Aerosol Research     

Particle charge-size distribution measurement using a differential mobility analyzer and an electrical low pressure impactor

We introduce a particle charge-size distribution measurement method using a differential mobility analyzer and an electrical low pressure impactor in tandem configuration. The main advantage of this type of measurement is that it is suitable for a wide range of particle sizes, from approximately 30 nm up to a micrometer, and for high charge levels, which have been problematic for previously used methods. The developed charge measurement method requires information on the particle effective density, and the accuracy of the measurement is dependent on how well the particle effective density is known or estimated. We introduce the measurement and calculation procedures and test these in laboratory conditions. The developed method has been tested using narrow and wide particle size distributions of a known density and well-defined particle charging states. The particles have been produced by the Singly Charged Aerosol Reference (SCAR) and an atomizer and charged with the previously well-characterized unipolar diffusion chargers used in the Nanoparticle Surface Area Monitor (NSAM) and in the Electrical Low Pressure Impactor (ELPI+). The acquired charge-size distributions are in good agreement with the reference values in terms of the median charge levels and widths of the charge distributions.

Copyright © 2017 American Association for Aerosol Research     

Aerosol Charge Fractions Downstream of Six Bipolar Chargers: Effects of Ion Source,Source Activity,and Flowrate

Bipolar diffusion charging is used routinely in aerosol electrical mobility size distribution measurements. In this study, aerosol charge fractions produced by six bipolar chargers (neutralizers) were measured using a tandem differential mobility analyzer system. Factors that were studied include the type of ion source (²¹⁰Po, ⁸⁵Kr, ²⁴¹Am, and soft X-ray), source activity, charger design, and aerosol flowrate. It was found that all six types of neutralizers achieve stationary state charge distributions when the source activity is sufficiently high. For ²¹⁰Po neutralizers with an initial radioactivity of 18.5 MBq (0.5 mCi), stationary state charge distributions are achieved when the source is less than 3.25 years old (residual activity no less than 0.0527 MBq). Stationary state was achieved for ⁸⁵Kr neutralizers having residual radioactivity greater than 70 MBq. Source activities of ²⁴¹Am and soft X-ray neutralizers are discussed. Aerosol charge fractions for six neutralizers remain reasonably invariant over a wide range of flowrates. The positive charge fractions achieved by the soft X-ray neutralizer are higher than those by the other five neutralizers using radioactive sources while negative charge fractions for all neutralizers studied are all in a similar range. This study also raises questions about bipolar charging fractions used for data inversion in some scanning mobility particle spectrometer (SMPS) systems, and underscores the need to better understand bipolar charging to achieve more accurate measurements of particle size distributions.

Copyright 2014 American Association for Aerosol Research     

Particle size and chemical composition effects on elemental analysis with the nano aerosol mass spectrometer

In the Nano Aerosol Mass Spectrometer (NAMS), particles are irradiated with a high energy laser pulse to produce a plasma that quantitatively disintegrates each particle into positively charged atomic ions. Previous work with this method used electrodynamic focusing and trapping of particles 30 nm dia. and below. In the current work, an aerodynamic focusing inlet was used to study particles between 40 and 150 nm dia. The distribution of atomic ion charge states was found to be particle size dependent, shifting toward lower charges with increasing size. This shift also affected the calibration by which elemental composition was determined from atomic ion signal intensities. Size independent calibration could be achieved by restricting the analysis to particles that gave more than 90% of the total signal intensity as multiply charged ions. This approach worked best for particles smaller than about 100 nm dia. since most spectra met this criterion. For the nanoparticles studied, the elemental mole fractions of Group I and II metals, halogens, and low atomic mass nonmetals could be determined within 10% or less of the expected value when the mole fraction was at the 1% level or greater. Some transition and heavy metals could not be quantified, while others could. Quantification appeared to be dependent on the ability of the element to be vaporized. Elements with high melting and boiling points gave particle mass spectra similar to those obtained by laser desorption ionization—mostly singly charged ions with relative intensities strongly biased toward atoms with low ionization energies.

Copyright © 2017 American Association for Aerosol Research     

Calibration of a particle mass spectrometer using polydispersed aerosol particles

Routine calibrations of online aerosol chemical composition analyzers are important for assessing data quality during field measurements. The combination of a differential mobility analyzer (DMA) and condensation particle counter (CPC) is a reliable, conventional method for calibrations. However, some logistical issues arise, including the use of radioactive material, quality control, and deployment costs. Herein, we propose a new, simple calibration method for a particle mass spectrometer using polydispersed aerosol particles combined with an optical particle sizer. We used a laser-induced incandescence–mass spectrometric analyzer (LII-MS) to test the new method. Polydispersed aerosol particles of selected chemical compounds (ammonium sulfate and potassium nitrate) were generated by an aerosol atomizer. The LII section was used as an optical particle sizer for measuring number/volume size distributions of polydispersed aerosol particles. The calibration of the MS section was performed based on the mass concentrations of polydispersed aerosol particles estimated from the integration of the volume size distributions. The accuracy of the particle sizing for each compound is a key issue and was evaluated by measuring optical pulse height distributions for monodispersed ammonium sulfate and potassium nitrate particles as well as polystyrene latex particles. A comparison of the proposed method with the conventional DMA-CPC method and its potential uncertainties are discussed.

Copyright © 2018 American Association for Aerosol Research